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(CNN) -- Louisiana officials Monday urged the hundreds of thousands of people in the state who fled Hurricane Katrina to stay where they are.

"It's too dangerous to come home," Gov. Kathleen Blanco said at a late afternoon news conference in Baton Rouge.

"The roads are flooded, the power is out, the phones are down and many trees are down. So chances are, if you tried to come in, you wouldn't be able to get your vehicle in. ...

"Please, I'm begging for patience," she said. "We are working hard to get you home, but not until it is safe."

The governor said she had ordered state police to block re-entry routes to all but emergency workers.

A public health expert said New Orleans residents who return to their homes would face "a wilderness" without power and drinking water that will be infested with poisonous snakes and fire ants. (Watch surging floodwaters almost swallow houses)

"We would really encourage people not to come back for at least a week," said Ivor van Heerden, deputy director of the Louisiana State University Hurricane Center and director of the Center for the Study of Public Health Impacts of Hurricanes in Baton Rouge.

Van Heerden ticked off the problems anyone returning to the city would find: "no sewage, no drinking water, contamination, threat of rapid increase in mosquitoes, roads are impassible, downed power lines everywhere, trees, debris from houses in the roads, no way to go shopping, no gas."

The water also has dislodged fire ants and thousands of snakes -- including poisonous water moccasins -- from their homes.

"If you came back, you would be coming literally to a wilderness," he said. "Stay where you are, be comfortable; nothing's going to change. If your house is gone, it's gone. If you come back in a day or a week, it's not going to make any difference."

The storm passed just east of New Orleans, straining the system of levees and pumping stations that protect the low-lying city, about 70 percent of which is below sea level. (Full story)

The governor said the full extent of the damage in southeast Louisiana remains unknown because it is still too dangerous for emergency teams to get to some areas.

Power is down and phones are out across the region, and authorities have not been able to put aircraft up to survey the devastation, she said.

Extensive damage from wind and water has been reported in Orleans, St. Bernard, Plaquemines, Jefferson, St. Tammany and Washington parishes, Blanco said.

There are "lots and lots of folks whose homes are no longer habitable -- roofs off, in some cases totally destroyed, and these people are now phoning in and asking to be rescued," Van Heerden said.

More than 50 people in the New Orleans area were rescued from flooded neighborhoods, according to a spokesman for the state's Homeland Security and Emergency Preparedness agency.

Lt. Kevin Cowan said the state Department of Wildlife and Fisheries sent 30 boats to the hardest-hit parts of the metro area, the city's 9th Ward and neighboring St. Bernard Parish.

Two dozen more boats were sent to hard-hit areas south of the Superdome and six were sent to Metairie, in Jefferson Parish, to carry out nursing home residents, he said. (Watch video of a helicopter rescue)

Water levels could be "anywhere from two feet to 10 feet" in those areas, Cowan said.

Other rescue efforts were going on in St. Tammany Parish, along the Mississippi state line, he said.

Van Heerden said some places in New Orleans have 8 or 9 feet of standing water and that he had been told that low pressure in the city's water supply means "they've got leaks."

He said he has received reports that the same areas of the city that flooded when Hurricane Betsy nearly landed a direct hit on New Orleans in 1965 have been flooded again, only more so.

Still, the impact of Katrina could have been far worse.

"Our biggest fear was that the storm would keep going west [while in the Gulf], which would have caused the catastrophic flooding of New Orleans," Van Heerden said.

"Very fortunately, the storm moved to the east and also dropped in strength a little. This was just enough to make that fairly large difference in the surge, so we did not have huge areas of New Orleans flooded."

Appearing at the news conference with Blanco, the director of the Federal Emergency Management Agency, Michael Brown, said President Bush would sign a federal disaster declaration for Louisiana.

"My guarantee to you is that FEMA will stay here as long as we are needed to help you in every way possible that we can help you," he said.